1. Field of the Invention
The present invention relates to an induction heat treatment method, an induction heat treatment installation and an induction-heat-treated product, and more particularly, to an induction heat treatment method, an induction heat treated installation and an induction-heat-treated product wherein a treatment object is heated through induction heating to be heat-treated.
2. Description of the Background Art
An induction heat treatment furnace is advantageous over a controlled atmosphere heat treatment furnace in that the working environment of the heat treatment furnace is clean and that the products of small lots can efficiently be treated in a short time period.
Generally, in induction hardening of steel, the quenching is actually conducted while varying the quenching condition of electric power and time, and the quality of the treated product is checked to empirically set the quenching condition. In this case, the quenching condition must be set every time the type of treatment object is changed, and therefore there is a problem that it is troublesome to set the quenching condition. This problem is attributed to the difficulty in conducting heat treatment by control based on the heat treatment condition of temperature and time (the temperature control) in induction hardening.
Further, also from the viewpoint of controlling steel structure forming the treatment object, it is desirable that quenching is conducted based on the temperature control. However, in induction hardening, there are technical difficulties in the temperature measuring method and in the high-speed temperature control, and in the current state quenching based on the temperature control is not employed.
The reason why measuring temperature in induction hardening is difficult lies in the fact that, being different from heating through an atmosphere, the treatment object is directly heated by induction heating and therefore temperature must be measured directly from the treatment object, and that it is difficult to place a contact type thermometer since an induction heat treatment installation is often provided with a drive mechanism for moving the treatment object for homogeneously heating the treatment object.
Addressing such problems, use of a non-contact type thermometer such as a radiation thermometer may be contemplated. However, there has been a problem that a conventional radiation thermometer is slow in response speed and therefore not suitable for measuring the temperature of a treatment object in induction heat treatment. The increased signal output speed of a radiation thermometer and the improved temperature measuring precision based on the emissivity of a treatment object in recent years may provide possibility in high-speed temperature control using the radiation thermometer.
Even when induction hardening based on the temperature control is possible, as a result of conducting induction hardening that basically is partial heating, the temperature is uneven through the treatment object. Accordingly, the heat treatment quality may vary among portions in the treatment object. This poses a problem in applying induction hardening to through hardening (which hardens the treatment object evenly from the surface layer to the inside). In particular, a thick treatment object exhibits great temperature unevenness, and therefore it is likely to be involved with the problem. When treatment object cannot be heated homogeneously, some portions sufficiently heated may satisfy the desired heat treatment quality, while some portions not sufficiently heated may not.
In order to solve such problems, it may be contemplated to set sufficient heating time to thereby homogenize the treatment object in temperature by heat conduction. It may also be contemplated to use a power supply with relatively low frequency so that the magnetic flux deeply enters into the treatment object to achieve even heating. However, both of the methods have the same problem to be solve as to how the sufficient heating time can be determined.
On the other hand, a tempering process of steel is provided to a quenched product for improving the toughness and dimensional stability. In many cases, the condition of tempering is determined by temperature for each steel type, and generally the holding time is more or less varied according to the thickness of the products, since the effect of tempering is dominantly affected by the temperature rather than the holding time. In order to reduce the time of tempering process, a tempering process of high temperature and short time may be contemplated. Additionally, it is known conventionally that the effect of tempering can be calculated from tempering temperature T and tempering time t using tempering parameter P expressed by the following equation (see J. H. Hollomon and L. D. Jaffe, Trans. Met. Soc. AIME, 162, 1945, pp. 223-249).P=T{log(t)+C}
(C: constant)
The equation expresses the temperature and time required for attaining the desired quenching effect. The following equations expressing the relationship among material strength, tempering temperature and tempering time are known as more practical equations (see Japanese Patent Laying-Open No. 10-102137).X=1−exp{−(kt)N}K=Aexp(−Q/RT)M=M0−(M0−Mf)X 
(X: rate of change in mechanical properties, k: reaction rate coefficient, t: tempering time, N: time index, A: vibration factor term, Q: activation energy, R: gas constant, T: tempering temperature, M: hardness after tempering, M0: hardness after quenching, Mf: hardness of raw material)
Although the effect of temperature and time on tempering can be known from the equations, generally a tempering process of high temperature and short time is not performed in the current state, in order not to invite uneven tempering in a heat treatment furnace. In many cases, a large quantity of treatment objects are processed at once in tempering. Accordingly, depending on the shape (thickness), processed quantity and the like of the treatment objects, the heating state in the heat treatment furnace may vary. Accordingly, as the heat treatment condition for tempering a large quantity of treatment objects at once, often a condition of relatively low temperature and long period is employed, according to which heating state is less likely to vary.
There is an induction heat treatment with which heat treatment of products is easily conducted individually, as a processing method suitable for tempering at high temperature and in a short time. As compared to tempering using a normal controlled atmosphere furnace, rapid heating tempering using induction heat treatment (induction tempering) is reported to be capable of providing a high-performance material property (see Kawasaki et al, “Iron and Steel”, vol. 74, 1988, pp. 334-341 and pp. 342-349). However, induction tempering is not a very common tempering method. The reason may lie in the fact that temperature control is difficult in induction tempering and setting of the heat treatment condition is troublesome, similarly to the induction hardening.
As above, while the heat treatment such as quenching and tempering using induction heating has various advantages, it involves the problems that heat treatment based on the temperature control is difficult, and that troubles and experiences are required in setting a condition for stabilizing the quality of the treatment object. Further, it is required to reduce the manufacturing costs of induction-heat-treated products as treatment objects and to stabilize the quality thereof by solving the aforementioned problems to make the heat treatment condition setting easier.